The present invention relates generally to improved compact hybrid actuators, and, more particularly, to an improved fluid pump that has an electrically-powered solid-state driver arranged to modulate the volume of a fluid chamber, and has a driven valve operatively arranged to control the flows of fluid with respect to the pump chamber. The pump driver and driven valve may be operated at frequencies greater than those attainable by oscillatory pumps with passive valves in the prior art.
The present invention relates to a new type of hybrid actuator that combines some of the most advantageous features of both hydraulic and electric actuators. In particular, it is well established that electrically-powered actuators (commonly referred to as electro-mechanical actuators, or xe2x80x9cEMAxe2x80x9ds) can be made small, lightweight and powerful, resulting in very high power density. They are also simple to install, service and replace because the power supply to them comprises only electrical cables. However, when an electrically-powered actuator (e.g., a motor-driven ball-screw, etc.) malfunctions or breaks, it will typically jam in the failed position, resulting in catastrophic failure of the system because of consequent loss of control authority.
On the other hand, hydraulic actuators offer the capability of being able to fail gracefully and predictably. For example, if a hydraulic actuator is used to control an aircraft control surface, a failure may result from leakage of hydraulic fluid, and therefore loss of controllable function, but the actuator will not seize in position. Hence, the aircraft may continue to be xe2x80x9cflyablexe2x80x9d by means of redundant actuators. Instead of locking in any particular position, the control surface can be made to return to a xe2x80x9cneutralxe2x80x9d position by the aerodynamic forces acting on it. Its return to neutral and subsequent movement may also be passively damped by the action of pistons in cylinders, and hydraulic fluid remaining in the system.
However, hydraulic systems must be supported by an extensive and complex mechanical infrastructure of central pumps, manifolds and tubing. Because of the large number of joints and fittings typically used, these systems are susceptible to corrosion and leakage, and are therefore maintenance intensive. In addition, maintenance and replacement is complicated by the need to bleed the system every time a hydraulic unit, such as an actuator, is removed from the system for service.
The hybrid actuator broadly comprises an electrical power supply, such as the electrical buss of an aircraft or submarine, power-conditioning electronics, an induced-strain material driven pump, and an output ram. The improved actuator employs a novel, compact, very high frequency pump that is collocated with the actuator to eliminate much of such hydraulic infrastructure. The pump is powered electrically, and the only connections to the actuator are electrical leads. Because the actuator mechanism itself is inherently hydraulic (although fed by a very small local pump in lieu of the previous large, complex central hydraulic system), it has the operational advantages of a conventional hydraulic actuator, discussed above. Because the pump is very small and light, the overall actuator (comprising the pump and output ram) has a very high power density.
Actuators that employ electrically-driven local pumps to directly supply pressurized hydraulic fluid to an actuating ram are known in the prior art. One fairly mature class of such actuators are known as Electro-Hydrostatic Actuators (xe2x80x9cEHAxe2x80x9ds). These actuators generate pressures using small, very high speed, multi-piston pumps driven by brushless DC motors at speeds up to 20,000 RPM. To control the direction and extent of ram motion, the operation of the motor is reversed to pump fluid to one side or the other of the ram. Typical applications include movement of control surfaces on fighter aircraft. While useful for large-scale large-force actuation, EHAs cannot be readily scaled down to applications below about 5 horsepower, primarily because the miniaturized piston pumps approach the limits of achievable tolerances and manufacturability at this size. Consequently, leakage becomes a larger percentage of total flow, and efficiency falls off unacceptably. In addition, the entire EHA system is relatively complex and has a high part-count when the structure of the electric motor, rotating piston pump and associated power conditioning electronics are considered.
There is increasing demand for small electrical actuators in applications requiring distributed structural control, such as morphing aircraft where the airfoil shape is adjusted to adapt to the operating environment and control demands. Similarly, in unmanned aircraft, there is a need for small electrically-powered actuators
The present invention provides a simpler, lower part-count alternative to such known EHAs, wherein the pump comprises a solid-state electroactive material, such as a piezoelectric material (e.g., lead zirconate titanate), controlling or modulating the volume of a small compression chamber with high-frequency inlet and discharge valve mechanization. Other active, or induced-strain, so-called xe2x80x9csmartxe2x80x9d materials could be employed, depending on the particular application. For example, a magnetostrictive material, such as Terfenol-D, can be used advantageously when the system must be operated over a wide temperature range. Military aircraft, for example, must use components that remain functional between about xe2x88x9265xc2x0 F. and about +265xc2x0 F. Electrostrictive materials, such as lead magnesium niobate, are more suitable for underwater applications, such as submarine rudder control actuators, where the operating temperature range is constrained over a narrow band.
While such materials provide a unique capability of generating very high force displacement in a lower part-count mechanism, the maximum strains obtainable are typically on the order of 1000 microstrains at best. Moreover, the materials themselves are generally quite dense (e.g., lead- and iron-based formulations), and thus must be operated at high frequency (e.g., on the order of 1 kHz-10 kHz) to achieve high power density. In a practical pump, the resulting pulsating flow must be rectified by inlet and discharge check valves. However, higher frequencies in this range are well beyond the capabilities of existing, conventional, passive check valves, which are typically limited to around 50-150 Hz, even with higher performance valves. Accordingly, to implement the invention, entirely novel passive designs must be employed, or the valves must be driven actively at very high frequencies to match the compression cycles in the pump chamber.
Others have proposed to use such induced-strain solid-state xe2x80x9csmartxe2x80x9d materials to operate an electrically-driven pump. For example, U.S. Pat. No. 4,927,334 (xe2x80x9cEngdahlxe2x80x9d) discloses various constructions of magnetostrictive rods displacing pistons to produce a pumping device. However, Engdahl generically indicates the necessary check valves by conventional symbols, and does not address their specific construction.
International Patent Application No. PCT/US97/15608 (Publication Number WO 98/11357), assigned to Etrema Products, Inc., discloses a magnetostrictively-driven pumping element in combination with magnetostrictively-driven inlet and exhaust poppet valves, a magnetostrictively-driven four-way directional control valve, and a piston/cylinder actuator. While this device recognizes the need for valve elements that can operate at the same frequency as the pump element, it does not adequately address the problem of providing reasonable valve openings using microstrain actuating elements, other than to suggest that mechanical motion amplifiers might be provided.
A disclosure of the University of South Carolina Office of Technology Transfer, OTT ID No. 97152 by Victor Giurgiutiu, shows a solid-state induced-strain pump with inlet and outlet xe2x80x9cvalvingxe2x80x9d elements, termed xe2x80x9cfluid diodesxe2x80x9d, that depend on the difference in dynamic flow impedance due to flow direction through a diffusing nozzle. Such elements have adequate frequency response, but leakage limitations.
Before the dominance of semiconductor electronic components, a large number of xe2x80x9cfluidicxe2x80x9d devices were proposed and developed as elements for use in logic circuits for computation purposes. One such device was invented and patented in 1925 by Dieter Thoma and is referred to as the xe2x80x9cThoma counterflow brakexe2x80x9d. It acts as a unidirectional element (i.e., a diode) for DC flow control by porting fluid into a vortex chamber either through the central hole of the vortex (i.e., the easy direction), or through an entry pipe tangential to the vortex (i.e., the direction of high resistance). Because the actual experimental diodicity of the simple vortex diode is rather low, its usefulness as a controller of mass flow is rather limited, especially when system considerations prohibit any backflow.
In 1969, C. A. Kwok invented a two-terminal device consisting of two triodes (i.e., a vortex diode with a second side port and grounded center hole) in series, which he called a xe2x80x9cdouble vortex diodexe2x80x9d. This device has large diodicities (i.e., flow forward relative to backward) and can, in principle, act as an effective unidirectional two-terminal element. However, because Kwok""s device functions primarily by dumping fluid out of its grounded center ports in the difficult direction, it is not useful as a mass driver.
Thus, these various references, either individually or collectively, teach the use of such electrically-controlled induced-strain pumps, together with actively-controlled two-way valves, to generate hydraulic power proximate the site of its usage, such as near a flight control surface. However, upon information and belief, while the broad concept may be old, these references do not teach certain improvements to the basic system that are disclosed and claimed herein.
The present invention provides an improved electrically-powered solid-state induced-strain driven pump in combination with either of two alternative valving mechanisms to deliver controlled flow or controlled pressure to a piston/cylinder actuator.
Two different valving mechanisms can be used to enable the rapid inlet and discharge flow cycles to and from the compression chamber. An active mechanism is described which uses a motor-driven rotating valve member with fluid paths to alternately align discharge and inlet ports with corresponding openings in a housing, in synchronism with the expansion and contraction of the pump chamber, respectively, so as to form one or more three-way valves for directing the pump flow. A passive mechanism is also described which exploits a particular structure with asymmetric forward and reverse fluid paths to produce a highly effective unidirectional xe2x80x9cfluid diodexe2x80x9d check-valve equivalent at each of the two chamber ports.
In order to rectify the high-frequency low-displacement motion of the solid-state piston pump to produce flows to a low-frequency high-displacement actuator, an effective, high-speed valve is required. During the compression phase of the pump piston""s travel, this valve must open the compression chamber to the pump""s pressure port. The pressurized fluid is then discharged to supply flow to an accumulator or actuator cylinder. As the piston reaches the point of furthest extension and reverses direction, the valve must close the passage to the pressure port and open the compression chamber to the return port. Hydraulic fluid is then drawn or ingested through the return port to refill the compression chamber, and the cycle repeats. In this way, a net positive flow from pressure to return is obtained from the oscillatory motion of the pump piston.
In a typical single-acting pump, this valving is provided by passive spring-loaded flow-operated check valves. As pointed out earlier, such valves cannot operate effectively at the frequencies required for an effective solid-state pump driver.
A similar problem would seem to exist in high-speed rotary piston pumps, such as used in a typical EHA, since the individual swashplate-driven pistons are oscillated at frequencies of the same order. However, such pumps use rotating valves driven by the same shaft as the swashplate and mechanically phased to the piston motion to provide the required pressure and return connections. The present invention effectively removes the swashplate piston drive of an EHA rotary pump, and replaces it with a solid-state piston drive, but retains the motor-driven rotating valve. By utilizing the proven rotary valve mechanization, high speed actuation of individual valves is achieved.
The valve-driving motor, of course, can be much smaller than a pump-driving motor, but the necessary electronic speed control driver is essentially the same in function, although only required to maintain a speed consistent with the pump driver frequency. The valve is no longer mechanically synchronized with the piston motion. Rather, the usual DC brushless motor position feedback signal can be used to provide phase synchronization of the pump current driver.
This feature can also be used to advantage in one form of the invention (e.g., the second embodiment, shown in FIG. 2) in which the solid-state driven pump is connected through the driven rotary valve directly to an actuator cylinder, instead of being used to charge a high-pressure accumulator to supply pressure to an electrohydraulic servovalve that controls flow to an actuator (e.g., as in the first embodiment, shown in FIG. 1). In this second embodiment, position feedback from the actuator piston is compared with a position command to generate a closed-loop error signal. Pump output flow is made proportional to this error signal by modulating the amplitude of the constant frequency current pulses applied to the solid-state pump driver. When the error signal passes through zero and increases with changed polarity, the current pulse amplitude is increased proportionately, but with its phase shifted by 180xc2x0 with respect to the rotating valve position reference, much as a modulated AC carrier signal would be.
In this case, the rotary driven valve consists of a three-way valving section which connects the pump compression chamber alternately to the extend or retract sides of the actuator cylinder. When the pump drive current pulses are in phase with the valve position, flow out of the pump is connected to the extending side of the actuator, and flow back into the pump is connected from the retracting side of the cylinder, so that the actuator piston rod extends. When the pump drive current pulses are shifted by 180xc2x0, the converse is true, with flow out of the pump being connected to the retract side of the actuator so that the piston rod retracts.
With parenthetical reference to the corresponding parts, portions or surfaces of the first disclosed embodiment, merely for purposes of illustration and not by way of limitation, the present invention provides, in one aspect, an improved fluid pump for use in a compact hybrid actuator (20).
This improved fluid pump broadly comprises: at least one electrically-powered solid-state pump driver (21) having a displacement element (22); a variable-volume fluid chamber (23), the volume of which is modulated by the position of the displacement element relative to the body; a first power controller (49) operatively arranged to selectively supply an alternating current to the pump driver; a driven valve (24) operatively arranged to control the flows of fluid with respect to the pump chamber, the driven valve having a body (28) provided with a plurality of ports (25, 26) and having a member (29) movable relative to the body to modulate the opening of the ports so as to form at least one three-way valve; a valve driver (31) operatively arranged to move the member relative to the body; and a second power controller (50) electrically coupled to the first power controller and operatively arranged to control the phase of the pump driver in synchronism with the phase of the valve driver; whereby the driven valve will allow fluid to be discharged from the chamber to the outlet port when the pump chamber contracts, and will permit fluid to be drawn into the pump chamber through the inlet port when the pump chamber expands.
As used herein, the expression xe2x80x9celectrically-powered solid-state pump driverxe2x80x9d refers to a pump driver that includes an electrically-powered induced-strain xe2x80x9csmartxe2x80x9d material, such as a magnetostrictive material, an electrostrictive material, a piezoelectric material, or the like.
In the preferred embodiment, the driven valve has a moving member that is arranged to be rotated relative to the body. The valve driver may be a motor (e.g., an electric motor, a hydraulic motor, or the like) having a rotatable output shaft that is coupled to the moving member.
The current supplied to the pump driver may be an alternating current (e.g., a sinusoidal waveform, etc.), or may be in the form of a series of current pulses. These may be rectified pulses, with phase being used to control the direction and amplitude being used to determine the magnitude of such flow.
The invention may further comprise a hydraulic actuator (e.g., such as a conventional fluid-powered piston-and-cylinder arrangement, etc.) that is operatively arranged to communicate with the driven valve. If desired, one or more accumulators may be operatively arranged between the driven valve and the actuator. A flow-control valve (e.g., an electrohydraulic servovalve, etc.) may be operatively arranged between the driven valve and the actuator for controlling the flow of fluid with respect to the actuator. In another form, the output of the driven valve may be provided directly to the hydraulic actuator.
The first controller may be operatively arranged to supply current to the pump driver in phase with respect to the position of the rotatable member in the valve driver to supply such current 180xc2x0 out of phase with respect to the phase of the valve driver position to move the hydraulic actuator in the opposite direction.
In yet another form, the pump may comprise two of such pump drivers, with these pump drivers being operatively arranged so that the motions of the respective pistons oppose one another. This configuration has an additional advantage that the Newtonian forces attributable to the acceleration of the respective elements in these pump drivers may oppose and cancel one another, thereby reducing the overall vibration of the system.
In another aspect, the invention provides an improved fluid pump which broadly comprises: an electrically-powered solid-state pump driver having a displacement element; a variable-volume fluid chamber, the volume of which is modulated by the position of the displacement element; a first controller operatively arranged to selectively supply an alternating current to the pump driver; and two passive double-vortex valves (130), each having an inlet port and an outlet port, these valves being arranged to control the fluid with respect to the chamber such that one double-vortex valve will allow fluid to be discharged from the chamber to the outlet port when the chamber contracts and the other double-vortex valve will permit fluid to be drawn into the chamber through the inlet port when the chamber expands.
Here again, many of the details previously discussed may be applied to this form of the improved fluid pump. The vortex valve may be arranged to have a greater impedance to flow in one direction than in the other direction.
Accordingly, the general object of the invention is to provide an improved compact hybrid actuator.
Another object is to provide an improved compact hybrid actuator that is particularly adapted for use in xe2x80x9cpower-by-wirexe2x80x9d aircraft applications where it is desired to generate hydraulic power locally in the vicinity of its application and usage.
Another object is to provide an improved fluid pump having an electrically-powered solid-state pump driver provided with a displacement element, a variable-volume fluid chamber, the volume of which is modulated by the position of the element relative to the surrounding body, and a driven valve operatively arranged to actively control the flow of fluid with respect to the chamber.
Still another object is to provide an improved fluid pump that employs an oscillatory pump driver and valve that may be operated at frequencies in excess of 1 kHz.
These and other objects and advantages will become apparent from the foregoing and ongoing written specification, the drawings, and the appended claims.